Potentially science and technology, things that can be transmitted via radio. Maybe Mars could setup a hollywood style entertainment industry :)

As far as minerals, I really doubt there are any which would be cheaper to mine on mars, load on a rocket and transport back to earth, than to mine directly on earth. Even if you build a space elevator on Mars (which is much easier than building one on earth).

Maybe you could setup a 3 way trade. Even without the space elevator, it's cheaper to ship things from Mars' surface to basically anywhere in the solar system than it is from earth's surface.

So Mars could ship oxygen, water, CO2 filters, building materials, maybe even food to an asteroid station (whatever they can't source locally) and the asteroid station could ship whatever they mine from the asteroid to earth.

I think "self-sustaining" in the context means that if Earth dissapears, explodes, turns into grey goo, or otherwise becomes unavailable, the Mars colony stands a chance of surviving indefinitely.

Yes, it's a hugely ambitious goal.

"Self-sustaining" by definition means able to survive indefinitely without importing anything from Earth.

Just few things needed for that

- mining industry

- chemical industry with thousands of end products.

- biochemical industry (medicines, plastics etc.)

- microprocessor fab and IC fabrication

- indefinitely sustained biosphere

- manufacturing industry for different high-end products, including rockets, instruments etc. and ability to maintain them.

I think you need large city with maybe 500,000 people at minimum with current technology.

Just look at people living in the Arctic: everything is imported, basic necessities are expensive, and getting replacements could take weeks.

Now imagine that you're on a different planet, and there are only a few firms that CAN resupply, and they'll only do it if you can compensate them sufficiently.

Better hope the colony is built on a platinum and rhodium mine!